There are two main applications of data transmission by means of guided light waves:
long-distance optical transmission of data and telecommunications signals, PA1 short-distance optical transmission of data onboard aircraft and ships and in industrial installations. PA1 a "front" surface at which the connection is made, PA1 at least three hard, rigid cylindrical locating pins parallel to and in contact with one another along their side surfaces, their front ends being located in the vicinity of the front surface of the connector without projecting beyond the front face, and forming fibre-locating grooves between the facing portions of their side surfaces, PA1 lateral clamping means for urging the locating pins into contact with one another in the lateral direction, PA1 guide means for guiding the set of locating pins so as to act as a guide for optical fibres located in the locating grooves, PA1 a base providing on an upper surface thereof, a hard, rigid plane base surface providing a reference plane on which the locating pins are located, PA1 means for clamping the locating pins against the base surface to maintain contact between the lateral surfaces of the pins and the base surface, PA1 an elastic pad for immobilising the fibres disposed adjacent the front face, and PA1 means for clamping the elastic pad onto the locating pins with sufficient force to deform the pad where it contacts the pins and/or the fibres so that is presses the fibres into the locating grooves formed between the pins. PA1 a rectilinear side support surface extending parallel to and above the base surface, PA1 a cylindrical main guide pin forming part of said guide means and located on said base surface parallel to the lateral plane of contact of said base surface with said set of locating pins, the front end of said guide pin being offset forwards or backwards relative to the front edge of the connector to permit plug/socket cooperation of the connector and an associated complementary connector, and PA1 means for clamping the guide pin against the base surface. PA1 two cylindrical stop pins located on said base surface parallel to and one on each side of said locating pins, said stop pins being clamped in the lateral direction by said lateral clamping means, and their front surfaces constituting the plane stop surfaces perpendicular to the pin axes, and PA1 means for clamping said pins against the base surface, PA1 the front ends of the locating pins being offset backwards relative to the stop surfaces so as to avoid said front ends coming into contact with the front surfaces of the locating pins of a complementary connector, the offset distance being less than 0.02 millimeters.
The first of these applications relies on optical cables in which each conductor or optical fibre is carrying data different from that carried by other fibres. Connections must be of the fibre-to-fibre type, and it is necessary to use a multicontact connector for connecting the individual fibres to one another.
The second application uses optical cables in which each conductor consists of a number of fibres. Within a given conductor, each fibre carries the same data as the neighbouring fibres. An overall conductor-to-conductor connection can be made, using an optical mixer which generally comprises a glass cylinder with two indices of refraction, the central section being aligned with the hexagonal array formed by the fibres of the cable. This produces a connection loss, which increases with decreasing values of the stacking coefficient, which is the ratio between the total cross-section of the fibre cores and the cross-section of the mixer core. This loss is generally of the order of 50%, or 3 dB, and is added to the losses caused by mechanical manufacturing tolerances and the mechanical tolerance in respect of the connecting members of the connector assembly. Thus a fibre-to-fibre connection is also advantageous in this second application.
The present invention is concerned with both of the main applications mentioned hereinabove.
The difficulty in producing fibre-to-fibre connectors results from the need to obtain an alignment of the fibres which is sufficiently accurate to give minimum coupling losses, of less than 1 dB, for example. If the core diameter of the fibres is, for example, 100 microns, the core being covered with cladding with a lower index of refraction, the difference being between 5.10.sup.-3 and 10.sup.-1, the connectors must be capable of locating the fibres within the following tolerances: the distance between the facing ends of the fibres must be less than 20 microns; the relative eccentricity must be less than 4 microns; and the relative inclination must be less than 1 degree. These tolerances are larger if the medium separating the two fibres is matched in terms of the index of refraction: 50 microns for the distance and 10 microns for the eccentricity, the maximum relative inclination being the same. In view of the small diameter of the fibres, it is difficult to remain within these tolerances.
Various attempts at solving the problem thus posed are now outlined. To facilitate the description the term "connector assembly" is used to designate the assembly of two cemplementary "connectors" which cooperate with one another to connect two cables, each connector being connected by a rear part to the end of one cable. The set of components in each connector which holds the end of a single fibre will be called a "contact". Thus each connector comprises as many contacts as there are fibre in the cable, its contacts being in the front part of the connector.
French published patent application No. 2,282,650 (Corning Glass Works) corresponds to U.S. Ser. No. 498,329 (Hawk) now abandoned and describes a "connector assembly for optical wave guides". This connector assembly uses two similar connectors each of which has a connection surface which cooperates with a connection surface of the other. On one of the connectors this surface is described as an upwardly facing surface while on the other it is described as a downwardly facing surface. These surfaces are applied against each other to establish a connection. In the first connector the fibres are fixed in a part of the connector which is behind the connection surface (i.e. to the cable end of it) and they extend halfway along the surface. They are laid along the rear half portions of V-shaped grooves formed between cylindrical pins which extend side by side along the connection surface to the frontend of the connection surface. These pins are pressed laterally against each other. It is essential for these pins to be made of flexible material since, when a connection is made, the connection surfaces are applied against each other in such a way that the front half portions of the pins of the second connector are applied in the fibre-bearing rear half portion of the V-shaped grooves of the first connector, thereby locking the fibres in their grooves. Likewise the fibres of the second connector (which extend half way along the rods of the second connector) are locked in the front half portions of the grooves of the first connector by the pins of the second connector and in line with the fibres of the first connector. If the pins were hard and rigid they would be in danger of damaging the fibres when the two connectors are clamped together. Unfortunately the flexibility of the pins results in inadequate lateral positioning of the fibres. Further, it is difficult to position the ends of the fibres exactly half-way along their respective pins, especially since the length of the flexible pins varies with the pressure applied thereto. Thus to avoid the possibility of the ends of the fibres overlapping and breaking each other it is necessary for a relatively large safety gap to be left between the ends of the fibres of the two connectors when assembled. There are thus drawbacks in using flexible material for the rods.
For this reason, it has been proposed elsewhere to locate each fibre in a channel formed by the facing lateral surfaces of three hard and rigid cylindrical locating pins. Highly accurate pins can be machined from hard steel at low cost. They are maintained parallel to one another and in contact at their lateral surfaces using appropriate clamping means. The fibre is held in the required position by the intermediary of an arrangement which guides the assembly of locating pins.
This guiding action can be much more accurate, as it is applied to an assembly which is much more robust and rigid and with much larger transverse dimensions than the fibre, which is accurately located along the axis of the assembly. The guide arrangement may itself consist of cylindrical pins of a hard rigid material, bearing on the outside surfaces of the locating pin assembly. An arrangement of this type is described in French patent of addition application No. 2,316,611, "Fibre locating device" and in the corresponding U.S. Pat. No. 4,050,783 (inventor: Andre TARDY).
The previously described arrangement is well suited to the fibre-to-fibre connection of multifibre cables when it is required to have the same hexagonal arrangement of contacts as applies to the fibres in the cable. This arrangement is not well suited to the production of connectors which can be stacked on top of one another for connecting a large number of multifibre cables in a small volume, maintaining the facility for readily modifying the connections between cables. Moreover, it is desirable to facilitate the connection of the connectors to the ends of the cables, in other words to provide for the rapid insertion of the set of fibres in the set of locating channels provided to receive them.
The present invention is intended to provide a fibre-to-fibre connector for multifibre optical fibre cables which can be easily connected to a cable and provides for low connection losses using readily stackable connectors, at moderate cost.
The present invention consists in a fibre-to-fibre connector for multifibre optical fibre cables, comprising:
The connector preferably further comprises:
The longitudinal location of the connector is preferably obtained by virtue of the fact that it comprises:
The invention will now be described, by way of non-limiting example only, and with reference to the accompanying diagrammatic drawings comprising FIGS. 1 to 20.
Components appearing in more than one figure are designated by the same reference.